The impact of auditory nerve experiments on cochlear implant design

Type II spiral ganglion afferent neurons drive medial olivocochlear reflex suppression of the cochlear amplifier

The dynamic adjustment of hearing sensitivity and frequency selectivity is mediated by the medial olivocochlear efferent reflex, which suppresses the gain of the ‘cochlear amplifier' in each ear. Such efferent feedback is important for promoting discrimination of sounds in background noise, sound localization and protecting the cochleae from acoustic overstimulation. However, the sensory driver for the olivocochlear reflex is unknown. Here, we resolve this longstanding question using a mouse model null for the gene encoding the type III intermediate filament peripherin (Prph). Prph^(−/−) mice lacked type II spiral ganglion neuron innervation of the outer hair cells, whereas innervation of the inner hair cells by type I spiral ganglion neurons was normal. Compared with Prph^(+/+) controls, both contralateral and ipsilateral olivocochlear efferent-mediated suppression of the cochlear amplifier were absent in Prph^(−/−) mice, demonstrating that outer hair cells and their type II afferents constitute the sensory drive for the olivocochlear efferent reflex.

The medial olivocochlear efferent reflex regulates cochlear outer hair cell-based amplification of sound energy. Here the authors show this dynamic control of hearing sensitivity is driven by sensory input from the outer hair cells and their type II spiral ganglion neuron innervation.

Effects of chronic stimulation on auditory nerve survival in ototoxically deafened animals

For almost 10 years, chronic stimulation has been known to affect spiral ganglion cell (SGC) survival in the deaf ear. However, the reported effects of chronic stimulation vary across preparations and studies. In this review, the effects of chronic stimulation on the deafened auditory periphery are examined, and variables that may impact on the efficacy of chronic stimulation are identified. The effects of deafening on the unstimulated peripheral and central auditory system are also described, as the deafened, unstimulated system is the canvas upon which stimulation-mediated effects are imposed. Discrepancies in the effects of chronic stimulation across studies may be attributable in large part to the combined effects of the deafening method and the post-deafening delay prior to chronic stimulation, which vary across studies. Emphasis is placed on the need to consider the natural progression of SGC loss following deafening in the absence of chronic stimulation, as the rate of SGC loss almost certainly affects both the efficacy of stimulation, and the impact of any delay between deafening and initiation of stimulation. The differences across preparations complicate direct comparison of protective efficacy of stimulation. At the same time, these differences can be used to our advantage, aiding characterization of the effects of different factors on the efficacy of chronic stimulation as a neuroprotective intervention.

The use of long-duration current pulses to assess nerve survival

This study investigated the usefulness of long-duration current pulses in assessing the status of the auditory nerve in ears with various degrees of retrograde neural degeneration. Guinea pigs were deafened with aminoglycosides prior to acute implantation of the cochlea and collection of electrically evoked auditory brainstem responses (EABRs). Analysis of wave I evoked with long-duration current pulses suggests that this evoked response is sensitive to degeneration of the peripheral processes of the auditory nerve. Correlations with spiral ganglion cell density show that EABR measures obtained with long-duration pulses are comparable to those previously established for estimating nerve survival. Further analysis indicates that this measure may provide unique information about the degenerative state of the nerve. Threshold EABR measures using long-duration pulses are evidently more place-specific than other measures. Also, results suggest that long-duration pulses may be sensitive to two phases of the degenerative process: degradation of the peripheral processes and subsequent degeneration of neural processes central to the spiral ganglion.

Estimation of surviving spiral ganglion cells in the deaf rat using the electrically evoked auditory brainstem response

A procedure was developed to record the electrically evoked auditory brainstem response (EABR) in the rat with sufficiently little stimulus artifact to permit systematic measurements of the first positive wave (P1), the compound action potential (CAP) of the auditory nerve. Our principal aim was to verify the theoretical prediction that maximum P1 amplitude is directly proportional to the number of excitable auditory nerve fibers. This was carried out in animals with graded lesions of the spiral ganglion induced by perfusion of the cochlea with different concentrations of neomycin. Two series of observations confirmed the theoretical prediction. Several measures of P1, including maximum amplitude, and slopes of the P1 and P1-N1 growth functions, were highly correlated with the number of surviving spiral ganglion cells. Correlation coefficients (r) ranged from 0.75 to 0.92. Amplitudes of the later waves exhibited much lower correlations with spiral ganglion cell counts. These findings suggest that measurement of the CAP in deaf humans, possibly as wave I of the EABR, should provide quantitative information about the status of the nerve, which could be useful in screening candidates for cochlear implants, prescribing the optimum device for individual patients, and determining how benefits derived from such devices relate to the condition of the auditory nerve.

Collaterals from lateral and medial olivocochlear efferent neurons innervate different regions of the cochlear nucleus and adjacent brainstem

Two populations of superior olivary neurons which project to different sensory cell regions in the cochlea also give off collateral projections to the ventral cochlear nucleus (VCN) and adjacent brainstem. To determine whether these VCN projections also have different targets they were characterized by selective retrograde amino acid transport. Retrograde transport of 3H-d-aspartate (D-ASP) selectively labeled the unmyelinated fibers and neurons of the lateral olivocochlear (OC) system including a dense collateral projection to the central VCN. Retrograde transport of 3H-nipecotic acid (NIP) labeled the myelinated fibers and neurons of the medial OC system, including collateral projections to the peripheral VCN, subpeduncular granule cells, and nucleus Y. Medial and lateral OC efferent collaterals thus innervate different regions of the CN. Lateral system collaterals overlap extensively with Type I spiral ganglion cell afferent input. They are well positioned to play a role in modulating afferent input to the central auditory system, as is the primary projection of these efferents to the cochlea. The medial system collaterals project near the recently described afferent projections of Type II spiral ganglion cells. The medial system collaterals may therefore be related to the function of outer hair cells, as the medial system primary axons appear to be in the cochlea.

Estimation of surviving spiral ganglion cells in the deaf rat using the electrically evoked auditory brainstem response

A procedure was developed to record the electrically evoked auditory brainstem response (EABR) in the rat with sufficiently little stimulus artifact to permit systematic measurements of the first positive wave (P1), the compound action potential (CAP) of the auditory nerve. Our principal aim was to verify the theoretical prediction that maximum P1 amplitude is directly proportional to the number of excitable auditory nerve fibers. This was carried out in animals with graded lesions of the spiral ganglion induced by perfusion of the cochlea with different concentrations of neomycin. Two series of observations confirmed the theoretical prediction. Several measures of P1, including maximum amplitude, and slopes of the P1 and P1-N1 growth functions, were highly correlated with the number of surviving spiral ganglion cells. Correlation coefficients (r) ranged from 0.75 to 0.92. Amplitudes of the later waves exhibited much lower correlations with spiral ganglion cell counts. These findings suggest that measurement of the CAP in deaf humans, possibly as wave I of the EABR, should provide quantitative information about the status of the nerve, which could be useful in screening candidates for cochlear implants, prescribing the optimum device for individual patients, and determining how benefits derived from such devices relate to the condition of the auditory nerve.

Central trajectories of type II spiral ganglion neurons

Previous attempts to trace the central pathways of the thin axons from type II spiral ganglion neurons have been hampered by technical difficulties such as fading of the reaction product as distance increases from the injection site (Ryugo et al.: Soc. Neurosci. Abstr. 12:779, '86; Brown: J. Comp. Neurol. 260:591-604, '87). By using small rodents (gerbils and mice), which have short auditory nerves, we have succeeded in filling the entire central axon and terminals of type II neurons after peripheral injections of horseradish peroxidase. The general course of the type II fibers within the auditory nerve and cochlear nucleus is similar to that of type I fibers except that terminals from type II neurons are often found in regions of the cochlear nucleus that have high densities of granule cells.

The central projections of intracellularly labeled auditory nerve fibers in cats: an analysis of terminal morphology

The axons of physiologically characterized spiral ganglion neurons (type I) were stained throughout their arborizations in the cochlear nucleus by the intracellular injection of horseradish peroxidase (HRP). The tips of the axonal branches were marked by distinct swellings, ranging in size and shape from small boutons to large perisomatic ramifications. Electron microscopic analysis of such swellings revealed ultrastructural features characteristic of primary auditory synapses, consistent with the hypothesis that terminal swellings identifiable in the light microscope represent presynaptic endings. On the basis of light microscopic differences in size, these endings were organized into three categories. Endings of relatively small size (terminal boutons, free endings, boutons with filopodia, string endings, and small complex endings) composed 94% of all terminal endings. Within this category of small endings, there were predictable variations in relative size and regional distribution that related to the spontaneous discharge rate (SR) of the fiber. The endings of low and medium SR fibers (SR less than or equal to 18 spikes/second) were smaller on average than those of high SR fibers (SR greater than 18 spikes/second). Furthermore, there were more endings arising from the ascending branch than from the descending branch when comparing fibers of the low and medium SR group with those of the high SR group. There were not, however, obvious morphological features of this ending category that correlated with the characteristic frequency (CF, the pure tone frequency to which the neuron is most sensitive). A second category contained medium-sized complex endings, most of which formed axosomatic contacts. This category composed 4% of the population and was found in close proximity to the perikarya of globular, octopus, and spherical cells. The endings from low and medium SR fibers were smaller on average than those from high SR fibers. These endings did not vary in their parent branch distribution with respect to fiber SR, nor did they exhibit morphological features that correlated with fiber CF. The third category contained large complex endings (endbulbs of Held) and composed 2% of the ending population. Within the anteroventral cochlear nucleus, these large, complex endings made axosomatic contact with spherical cells in the anterior division and with globular cells in the posterior division. There were no systematic variations in ending size or branch distribution that correlated with fiber SR. There was, however, a relationship between ending size and fiber CF such that fibers having CFs below 4 kHz gave rise to the largest endbulbs.

Low-frequency sensorineural hearing loss. Brainstem response, speech and high-frequency audiometry

Four cases of acquired, unilateral sensorineural hearing loss are presented. The low-frequency octave threshold changes are considered compatible with the low-frequency tails of neurons with characteristic frequencies of 2, 4 and 9 kHz. Zero speech discrimination was found in two patients with high-frequency threshold preservation at and above 4 kHz. Normal auditory brainstem responses were obtained in the two patients with low-frequency hearing loss confined to the frequency bands below 2 and 4 kHz respectively. Only wave I could be reliably identified in the two patients with single frequency threshold preservation at 2 and 9 kHz, while both presented a second, low amplitude wave with latency corresponding to our normative values for wave V. The latter may represent a delayed and pathological wave III.